The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, it is desired that HDDs be able to store more information in their limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components.
The further miniaturization of the various components, however, presents its own set of challenges and obstacles. The development of microwave-assisted magnetic recording (MAMR) systems for enhancing the surface density of magnetic recording media has benefited higher density recording. In MAMR, in addition to a writing magnetic field emitted by a main magnetic pole, an alternating current (AC) magnetic field is applied to a recording medium from a spin torque oscillator (STO). Because the coercivity of a recording medium drops when an AC magnetic field is applied thereto, this renders high-quality recording more easily obtainable.
The STO is arranged between the main magnetic pole and a trailing shield. A conventional STO structure is typically comprised of the following layers: a main magnetic pole/spin-polarization layer/non-magnetic interlayer/oscillation layer/non-magnetic cap layer/trailing shield. In other conventional structures, an STO may be defined by a main magnetic pole/non-magnetic layer/oscillation layer/non-magnetic interlayer/spin polarization layer/trailing shield.
The spin polarization layer possesses magnetic anisotropy in the direction perpendicular to a film surface of the STO. The spin polarization layer is chosen such that when an electric current flows to the STO, the electron spin produced by the spin polarization layer has the same orientation as the spin polarization layer. These electrons impart a torque (“spin torque”) to the magnetization of the oscillation layer and, as a result, a magnetization rotation of the oscillation layer occurs. This magnetization rotation of the oscillation layer forms an AC magnetic field which is emitted by the STO.
There are some inherent problems with the use of recording heads having a conventional STO structure. On such problem is that while the writing magnetic field gradient is increased to produce a high signal-to-noise ratio (SNR), this typically necessitates a narrowing of the gap distance between the main magnetic pole and the trailing shield. However, the existence of the STO renders a narrowing of the gap to a width equivalent to or less than the STO film thickness problematic or impossible. When the writing magnetic field generated from the main magnetic pole is low, even if the recording is assisted by an AC magnetic field from the STO, the noise of the recorded signal pattern is increased and the SNR is lowered.
Another typical problem common to MAMR systems involves the reversal of the magnetic field polarity. Simultaneously with the reversal of the magnetic field polarity from the main magnetic pole to match the polarity of the recording bits, the magnetization direction of the spin polarization layer is reversed in typical MAMR systems. Because the STO is provided in the gap between the main magnetic pole and the trailing shield, the magnetization reversal of the spin polarization layer is produced by the magnetic field generated from the main magnetic pole. Accordingly, when the polarity of the recording bits is altered, the recording is performed in the sequence of main magnetic pole magnetization polarity reversal, spin polarization layer magnetization polarity reversal, and oscillation layer magnetization oscillation. As a result, following the completion of the main magnetic pole polarity reversal, there is a time delay until the oscillation layer magnetization attains stable oscillation. This results in a delay in the AC magnetic field generated by the STO with respect to the writing magnetic field generated by the main magnetic pole, which acts to preclude adequate recording assistance from occurring in the vicinity of the location of transition, and acts to preclude the production of a high SNR. Accordingly, in conventional MAMR systems, the higher the transfer rate during recording, the longer the relative time delay becomes.
Therefore, it would be beneficial to have a MAMR system which overcomes the problems associated with conventional MAMR systems.